This invention relates to the control of fluid flow and, more particularly, to a digital fluid flow control system.
Conventionally, a single element analog valve is employed to control the rate of flow in a fluid system. The essential components of such a valve are an orifice through which the fluid flows and a plug that is movable into and out of the orifice. The flow rate through the valve is determined by the extent that the plug blocks the orifice. There are also fluid control systems in the prior art that utilize multiple valve elements. They are, for the most part, digital flow control devices comprising a plurality of digital valve elements capable of assuming one of two stable states--open or closed. Generally, the orifice areas are weighted to follow a geometric progression of two.
Schmohl et al. U.S. Pat. No. 2,229,903, issued Jan. 28, 1941, discloses a digital valve in which two parallel laterally displaced conduits are connected by a plurality of cross ports distributed at different points along the length of the conduits. These cross ports have orifices with different areas and individually actuatable plugs blocking the orifices. It is taught that the orifices are selectively unblocked either individually or in combinations to control the rate of flow through the valve. In this way, many more different flow rates can be established by the valve than there are individual orifices.
Dufour U.S. Pat. No. 3,063,468, issued Nov. 13, 1962, discloses a plurality of valve elements stacked adjacent to one another along the direction of fluid flow. The valve elements take the form of discs that are individually rotatable into either of two positions. Apertures are arranged on the discs so that a different number of apertures of the discs are aligned for each combination of disc positions. The number of aligned apertures follows a geometric progression of two as successive discs are rotated. Fluid flows through the aligned apertures. Thus, the more apertures that are aligned, the more fluid flows through the system.
U.S. Pat. No. 3,072,146, issued Jan. 8, 1963, to T. Gizeski, is directed to a digital regulator valve in which transverse inlet and outlet manifolds are linked at different points along their lengths by conduits having digital control valves with orifice areas following a geometric progression of two. An upstream conduit feeds the inlet manifold, and the outlet manifold supplies a downstream conduit. A digital programmer controls the operation of the digital valve elements and therefore the rate of fluid flow through the system.
Ernyei U.S. Pat. No. 3,331,393, issued July 18, 1967, discloses a fluid flow control system employing balanced digital valve elements. The upstream conduit of the fluid system is connected to a first disc-shaped cavity and the downstream conduit of the fluid system is connected to second and third disc-shaped cavities located on either side of the first cavity. Each digital valve element cuts through the three cavities at a different point, having an orifice between the first cavity and the second cavity and an orifice between the first cavity and the third cavity. Two plugs mounted on the same rod control the fluid flow through the orifices of each valve element. The force exerted on one of the plugs due to the pressure drop across its orifice is balanced by the force exerted on the other plug due to the pressure drop across its orifice.
The prior art fluid flow control systems employing multiple digital valve elements suffer numerous shortcomings that are especially serious at high pressures and fluid flow rates. First, the valve elements are distributed along the direction of fluid flow in the system. As a result, the pressure drops across the valve elements differ from one another and are dependent on the states of the other valve elements. Correspondingly, the flow rates through the valve elements are also interdependent, i.e., not solely a function of the orifice areas. In a digital fluid flow control system this interdependence is manifested as a deviation in the flow rate from the nominal digital values.
Second, the problem of the formation of a vena contracta by the fluid passing through the digital valve elements is not met. At high flow rates, the vena contracta causes the effective orifice area of the valve element to become pressure dependent, thereby introducing a source of unlinearity into the fluid control system. In addition, the vena contracta frequently triggers cavitation, which causes pitting of the valve parts and inefficient operation.
Third, the valve elements of a digital fluid flow control system invariably have orifice areas that are weighted to follow a geometric progression of two. Accordingly, one-half of the maximum flow rate through the fluid flow control system is attributable to only one valve element. Full advantage is not then taken of the potential simplifications in valve design made possible by the fact the fluid is handled by a plurality of valve elements rather than a single element in an analog valve. Further, each time the most significant valve element is opened or closed, an extraordinary disturbance may be created in the fluid system because of the possible differences in opening and closing time of the different elements.
Fourth, it is difficult to design the digital valve elements themselves so the fluid rate in the open state remains constant in the course of prolonged use. Each valve element comprises a flow determining orifice and a plug that assumes one of two stable positions. When the valve element is open, the plug is positioned to seal the orifice. When the valve element is closed, the plug is positioned in the fluid stream in spaced relationship from the orifice so the orifice is unblocked. In the open state of the valve element, the extent to which the orifice is unblocked depends upon the position of the plug since the plug is in the fluid stream. As the moving parts of the valve element wear with prolonged use, the position of the plug in the fluid stream in the open state varies, and the flow rate varies accordingly. In other words, the plug tends to modulate the flow rate in the same manner as an analog valve.